Allogeneic hematopoietic cell transplantation (HCT) is a potentially curative therapy for blood related cancers including leukemia, lymphomas, and multiple myeloma. Its clinical success has been limited by the frequent development of severe and life-threatening acute graft versus host disease (aGVHD). Although monitoring of prognostic plasma biomarkers enables clinicians to stratify high-risk patients at onset of aGVHD for aggressive therapy, no drug has been specifically developed for aGVHD and subsequently approved to date. Among the aGVHD biomarkers, elevated levels of soluble STimulation-2 (sST2, ST2 is also named IL33Rc) is the most significant factor to predict steroid-resistant aGVHD that leads to non-relapse related death. sST2, functioning as a decoy receptor, traps IL-33 to reduce secretion of type-2 cytokines and contributes to overt pro- inflammatory type-1 immunity in aGVHD development. Our central hypothesis is that sST2 can be a therapeutic target and blockade of sST2/IL-33 interaction is a novel strategy to ameliorate aGVHD. To support the hypothesis, we have reported that peritransplant administration of ST2 neutralizing antibody in mice leads to decreased sST2 production and increased number of Th2 and Tregs cells post-transplantation resulting in alleviated aGVHD and improved survival. Motivated by the same objective, we recently completed a project to discover three chemical classes of small molecule ST2 inhibitors by employing high throughput screening and computational analysis. When evaluated in mouse aGVHD models, one compound produces the best outcome including improved survival, reduced plasma levels of sST2 and undiminished graft-versus-leukemia effect. The rationale of this study is that our lead compounds can be further optimized for pre-clinical therapeutic development including advancement to orally bioavailable agents, infeasible for antibody-based therapeutics. In this study, the three specific aims are: 1) To design and synthesize new analogs based on one lead compound with the aim of improving its potency, selectivity, and physicochemical properties for in vivo studies. 2) Determination of in vitro stability/toxicity of the inhibitors and their activities in the in vitro aGVHD assay. 3) Assessment of in vivo absorption, distribution, metabolism, and excretion (ADME)/Toxicity of the inhibitors and their efficacies in aGVHD mouse models. Current aGVHD therapy adopts drugs designed for other diseases and these drugs target non-specific effector T cells. Our innovative approach builds on the foundation of the first-in-class ST2 inhibitors discovered through our previous study to target the most significant prognostic biomarker for aGVHD. The significance of the proposed research is that the aGVHD-specific small molecule inhibitors obtained from this work will target appropriate effector T cells to increase efficacy with reduced toxicity. Our long-term goal is to develop oral therapeutic agents with which to treat aGVHD and other ST2/IL-33 axis mediated diseases.